Fm-cw doppler radar



Nov. l, 1966 R. w. WARD 3,283,321

FM-GW DOPPLER RADAR Filed Jan. 6, 1964 mi (n fw 3,283,321 IM-CW DOPPLER RADAR Raymond W. Ward, Northbrook, Ill., assigner to Motorola, Inc., Franklin Park, Ill., a corporation of Illinois Filed Ian. 6, i964, Ser. No. 335,717 3 Claims. (Cl. 343-14) This invention relates generally to a Iradar system and in particular to a continuous wave, frequency modulation system for obtaining the range and radial velocity of a moving target.

Continuous wave (CW) radar systems are in frequent use for `obtaining the velocity of targets by measuring the Doppler shift of a return signal from the moving target. Other forms of radar systems have been developed which will measure the range of a target but they will not distinguish a moving target from a stationary target without Icomplex equipment too bulky to be easily carried and used. It is desirable to combine the advantages of the different types of radar systems and to obtain from a simple compact CW frequency modulation (FM) radar system both range information and moving target information.

Accordingly, it is an object of this invention to provide an FM-CW radar system which is simple in construction and operation and which will provide information about moving targets located at a particular range.

Another object of the invention is to provide an FM-CW radar system which will provide information about moving targets at a plurality of ranges.

Another object of this invention is to provide an FM-CW radar system which will -measure the radial velocity of a moving target at a particular range.

A feature of this invention is the provision of an FM-CW radar system for obtaining range and moving target informaiton and having a mixer for mixing the transmitted and received signals to obtain a difference signal having frequency spectral components which are a function of the range to the target producing the received signal.

Another feature o-f this invention is the provision of an FM-CW radar system with a I-Iall multiplier for multiplying the detected received signal and a periodic local signal, and an audio filter for filtering the resultant signal, to produce an output signal having moving target information including the radial velocity of the Imoving target.

Another feature of this invention is the provision lof a CW radar system with a video lter to compensate for differences in the streng-th of the received signals due to di erent signal path lengths.

The invention is illustrated in the drawings wherein:

FIG. l is a block diagram of an FM-CW radar system incorporating the features of this invention;

FIG. 2 is a curve showing the frequency `of the transmitted and received signals of the radar systems as a function of time;

FIG. 3 is a curve showing the difference frequency output from the mixer as a function of time;

FIG. 4 illustrates the spectral density of the transmitted radar signal;

FIG. 5 illustrates the spectral density of the output of the mixer for a signal received from a single target; and

FIG. 6 illustrates the operation of the Hall multiplier used in this invention.

In practicing this invention a source of radio frequency energy, frequency modulated by a signal having a periodi-c function, is coupled to an antenna by means of a circulator. The energy thus coupled is radiated and is reiiected from both stationary and moving objects. The reilected radio frequency energy is received on the same antenna and coupled through the circulator to the receiver portion of the radar system.

In the radar receiver an attenuated portion of the transmitted signal is mixed with the received signal to produce a resultant signal having a frequency dependent upon the time taken by the received signal to travel to the target and return. The mixed signal is coupled to a Hall multiplier where it is multiplied by a sine wave signal having a frequency which is an integral multiple of the frequency of the modulating signal. The output signal of the Hall multiplier consists of a pair of frequencies equal to the sum and difference of the input frequencies. This output is coupled to an audio band pass filter which rejects the sum frequency. By proper choice of the frequency of the sine wave signal, dependent upon the range of the target about which information is desired, the frequency of the difference output signal from the Hall multiplier is reduced to zero if the tar-get is stationary. For moving targets the Doppler shift of the return signal produces a difference output signal from the Hall multiplier which has a frequency in the audio range. This audio frequency is coupled to a utiliza-tion circuit by a band pass filter. The frequency of the local signal determines the range at which the target is located and the audio frequency is a measure of the Doppler shift and thus a measure of the radial velocity of the target. If the local signal contains a plurality of frequencies, as for example a signal having the same periodic function as the modulating signal, a plurality -of range increments can be observed at one time.

An FM-CW radar system incorporating the features of this invention is illustrated in FIG. l. In this example a source of microwave energy 20 is frequency modulated by a signal of sawtooth wave form having a frequency FM, with a period equal to l/fm. This signal may be generated by square wave oscillator 2l and saw tooth generator 22. The output of the microwave energy source Ztl is coupled to directional coupler and by isolator 24. A small portion of the energy is coupled from directional coupler 25 to attenuator 26 and the remaining energy is coupled to circulator 28. Circulator 23 couples the energy to antenna where it is radiated. The radiated signal 31 is reflected from a target 32 and this reftected energy 34 returns to the antenna 30. The energy received by the antenna is coupled through circulator 28 to mixer 35 where it is mixed with the attenuated transmitted signal.

Referring to FIG. 2, the modulation of the source of microwave energy is shown as a plot of frequency vs. time. The transmitted energy is shown in curve 50. The received signal will vary in frequency with the same periodic function as the transmitted wave, but it will be delayed in time because of the time taken to travel from the antenna to the target and return. The received wave is shown in FIG. 2 as curve 51. FIG. 3 shows a plot of the difference frequency between the received and transmitted waves, the output of mixer 35, as a function of time. Curve 6i) of this gure shows the difference frequency produced by a stationary target.

FIG. 4 shows the power spectral density of the transmitted signal, and is a plot of amplitude vs. frequency. It can be seen that the transmitted wave has its power equally distributed at points separated by the modulating frequency, fm, over a frequency range equal to the 4frequency `deviation Af.

Curve 63 of FIG. 5 shows the power spectral density of the Aoutput of mixer 35 when the received signal is reflected from a single stationary target. Since the difference frequency output of mixer 35 is periodic, with a period of l/ fm, all the spectral components are located at integral multiples of fm. Since the frequency modulation, except at close range, has a large index of modulation most of theenergy from the target at rangev 1' will be inthe corresponding spectral line closest to that ran gc as i-s shown by curve 68 of FIG. 5. When'a moving target is encountered the difference frequency ;is displaced by the yDoppler shift as `shown by the dotted curve ,6l of v FIG. 3. ,This Dopplershift creates side ban-ds about the spectral linefcorresponding rto curves'fland 7l ofFlG., 5..

ReferringagaintoFIG. 1,:the outputfof themixer isv f amplified invideo amplifier 36 and coupled to video filter: 33. Video filter 38 removes the large ynegative difference. 'frequencies shown in FIG; 3, andequalizes thedifferences in the amplitude of the returning signals due to range differ-ence.

The output of'video filter iscouf' pled toy Hall multiplier 30; kA local input signal to Hall multiplier39 is obtained from a synchronized oscillaton 40. The output of'this oscillatordflis synchronized with f the square wave oscillator 2l by a monostable Shaper till.L

Synchronized oscillatore@ can be adjusted so that its outi put is a sine wave 'havinga frequency equal to an integral multiple of the frequency of'squarewave oscillator 2l, or

its output can contain a plurality of frequencies harmonically related to fm. 'An' example of this latter output l would be a periodic function similar to the periodic function modulating microwave source 2t). f

Han multiplie-r sa ris' Shown inr FIG. ,6. The Hau veiethe magnetic'fleld andthe current.

is obtained from oscillator 40ct FIG.. 1. AThe voutput voltage from the Hall element ,is coupled to band nassr` filter 43 and isa signal composed of the sum and diflerence frequencies of the inputs to the Hall multiplier.y

Consider .for example,a system'in which Af, the deviation of the CVV-signal, isequal to-7.5 megacyclesand fm is equal to 4 kil'ocycles. Each of the spectral lines are spacedy 4 lic. apart and represent 2G meter. range kincrements'. LA stationary target 500 meters away would cause the energyto be concentratedat the 25th spectralline or at kc. yBy adjustingthe sinewaveoscillator til to a frequency'equal'toy the ZSt-hliarmonic of fm, jor lOl) 'i kc.,the sum; frequencyfof theoutput of the Hall multii Aplier 39 .will be equal to 200 kc. and the` difference frequency would be equal to zero.- Both of .these frequencies yare outside the pass band of filter 43 and are thus rejected.v

A second stationary target '20 meters yfrom the first target would produce a difference signal from the Hall multiplier v equal to 4 kc which would also be outside the pass bandofffilter 43.3 Thus all stationary targets yproduce frequencies which arerejected by bandpass'lter 43.

- Assume the first target is moving ata velocity of 60 s miles an hour relative to the antenna of the system. At

a `frequency of 10 kmc.- rthis velocity will Lproduce a Doppler shift Iof 1800 cycles per second. Thus the energyk returned from the moving target would be contained in side bands at a frequencyof 100 kc., plus and minus 1.8 kc. The difference frequency between this signal andthe out-put of sine rwave oscillatorfitly would now be 1800 cycles vper second whichy is withinthe pass band of the filter 45. Thusamoving target at the range represented if bythe th harmonic of fm would bedetected. How- If the target is stationary,v the signal applied to Hallmultiples of fm. Band `pass lter i3 is constructed to reject DC. signals and signals having a frequency equal to or greater than fm. Thus -for a stationary target the sum frequencies being always greater than fm will be outside the vpass band of filter 43 and will be rejected. Also the difference frequencies of all stationary targets will be equal to Zero or equal to or greater than fm, and will also be rejected by the band pass filter d3.

If the signal is received from a moving target the Dop-pler shift will produce side bands which differ in frequency from the spectral line, corresponding to a stationary target at the same range as the lmoving target, by an amount dependent upon the velocity of the moving target. These side bands are illustrated in curves 70 and 71 of FIG. 5. By selecting a frequency for oscillator d@ so that the difference output of the Hall multiplier would be a direct current for a stationary target appearing at the same range as the moving target a signal will be developed with a frequency :depen-dent upon the velocity of the target. The signal from filter 43 is coupled to amplifier de and from the amplifier to indicator 4S. Indicator 4S includes a velocity meter to determine the radial velocity of the target by measuring the frequency of the signal from filter 43.

If the local signal from oscillator 46 contains a plurality of frequencies harmonically related to fm, moving targets at a plurality of ranges can be detected simultaneously. By using a signal containing a large number of harmonics a large area can be observed for moving targets. When a 4moving target is located, its range and radial velocity can be determined by switching oscillator dfi to a sine wave output. It should be noted that with a plurality of targets and a plurality of local signal frequencies only moving targets are detected, as in the case where a local signal of one frequency is used.

. ymultiplier 39 from video filterffl will be an integral .rnultiple off/11'.y iIf the local signalk from oscillator ellis a sinev wave, the sum and difference frequencies will be integral `ever, moving targets rat rangesA represented by other spectral lines will produce frequencies outside the pass bandy of filter 43 and wouldnotbe detectedy by the radar system;

Thus, a simple FM-CW radarsystem has beenshown f .vwhich will detect moving targets and reject stationary ones.; The range and radial velocity of thedetected target f can easily be obtained with this system.

1 claim: f f i `1. A continuous wave radar system including in `Com-r bination, firstmeans for generating ay -contiuuous wave radar signal having a :frequency which varies -as a periodic generating means for radiating said radar signal and receiving and translating reflected signals of the same, mixer means coupled to said antenna means and said first generating means for mixing said reflected signal and a portion of said generated signal to produce a difference signal having a frequency equal to the frequency difference between said mixed signals, second means coupled to said first generating means for generating a local signal having a period equal to the period of said periodic function, Hall multiplier means coupled to said mixer means and said second means, said Hall multiplier means being responsive to said difference signal and said local signal to produce output signals having frequencies equal to the sum and difference of said difference signal and said local signal, and band pass filter means coupled to said Hall multiplier means, said filter means being responsive to said output signals to reject frequencies thereof outside a predetermined pass band.

2. A continuous wave radar system including in combination, first means for generating a continuous wave radar signal, modulating means producing a sawtooth modulating Wave coupled to said first means for frequency modulating said radar signal, antenna means coupled to said first generating means for radiating said radar signal and receiving and translating reflected signals of the same, mixer means coupled to said antenna means and said first generating means for mixing said reflected signal and a portion of said generated signal to produce a difference signal having a frequency equal to the frequency difference between said mixed signals and a signal strength variation proportional to the frequency of said difference signal, video filtering means coupled to said mixer means to provide compensation for said signal strength variation, second means coupled to said first generating means for generating a local sine wave signal having a frequency equal to an integral multiple of the frequency of said sawtooth modulating Wave, Hall multiplier means coupled to said video filtering means and said second means, said Hall multiplier means being responsive to said difference signal and said local signal to produce output signals having frequencies equal to the sum and difference of said difference signal and said local signal, and band pass lilter means coupled to said Hall multiplier, said band pass lter means being responsive to said output signals to reject frequencies thereof outside of a predetermined pass band.

3. A continuous Wave radar system including in combination, modulating means for generating a linear ramp modulating signal having a first frequency, first generating means coupled to said modulating means and being responsive to said modulating signal to generate a continuous Wave radar signal having a second frequency which varies as a function of said modulating signal, antenna means coupled to said first generating means for radiating said radar signal and receiving and translating reflected signals of the same, mixer means coupled to said antenna means and said first generating means for mixing said reflected signal and a portion of said radar signal to produce a difference signal having a frequency equal to the frequency difference between said mixed signals, second generating means coupled to said modulating means for generating a local signal having a single fixed frequency of sinusoidal Waveform equal to an integral harmonic of said first frequency, said second generating means further including adjustable tuning means whereby a particular desired harmonic of said first frequency may be selected, signal multiplier means coupled to said mixer means and said second generating means, said signal multiplier means being responsive to said difference signal and said local signal to produce output signals having frequencies equal to the sum and difference of said difference signal and said local signal, and band-pass filter means coupled to said signal multiplier means, said filter means being responsive to said output signals to reject frequencies thereof outside a predetermined pass band.

References Cited by the Examiner UNITED STATES PATENTS 9/1952 Rust 343-9 5/1960 Glegg 343-8 

1. A CONTINUOUS WAVE RADAR SYSTEM INCLUDING IN COMBINATION, FIRST MEANS FOR GENERATING A CONTINUOUS WAVE RADAR SIGNAL HAIVNG A FREQUENCY WHICH VARIES AS A PERIODIC FUNCTION OF TIME, ANTENNA MEANS COUPLED TO SAID FIRST GENERATING MEANS FOR RADIATING SAID RADAR SIGNAL AND RECEIVING AND TRANSLATING REFLECTED SIGNALS OF THE SAME, MIXER MEANS COUPLED TO SAID ANTENNA MEANS AND SAID FIRST GENERATING MEANS FOR MIXING SAID REFLECTED SIGNAL AND A PORTION OF SAID GENERATED SIGNAL TO PRODUCE A DIFFERENCE SIGNAL HAVING A FREQUENCY EQUAL TO THE FREQUENCY DIFFERENCE BETWEEN SAID MIXER SIGNAL, SECOND MEANS COUPLED TO SAID FIRST GENERATING MEANS FOR GENERATING A LOCAL SIGNAL HAVING A PERIOD EQUAL TO THE PERIOD OF SAID PERIODIC FUNCTION, HALL MULTIPLIER MEANS COUPLED TO SAID MIXER MEANS AND SAID SECOND MEANS, SAID HALL MULTIPLIER MEANS BEING RESPONSIVE TO SAID DIFFERENCE SIGNAL AND SAID LOCAL SIGNAL TO PRODUCE OUTPUT SIGNALS HAVING FREQUENCIES EQUAL TO THE SUM AND DIFFERENCE OF SAID DIFFERENCE SIGNAL AND SAID LOCAL SIGNAL, AND BAND PASS FILTER MEANS COUPLED TO SAID HALL MULTIPLIER MEANS, SAID FILTER MEANS BEING RESPONSIVE TO SAID OUTPUT SIGNALS TO REJECT FREQUENCIES THEREOF OUTSIDE A PREDETERMINED PASS BAND. 